In the field of lighting, present developments aim to replace common incandescent or halogen lamps with so-called retrofit lamps, in particular using light emitting diodes (LEDs), where they are preferred over conventional lamps due to significant advantages such as increased energy efficiency and lifetime.
When retrofitting existing lighting systems with such lamps, it is typically required to adapt the retrofit lamp to the respective lighting system installed to allow proper operation, since a change in the setup or wiring of the respective lighting system to be retrofitted, for example installed in an office building, is not easily possible and would result in substantially increased cost of the overall retrofit process.
A particular example for the above mentioned retrofit application is the replacement of common halogen type lamps in low-voltage lighting systems. Such lighting systems typically comprise a power supply to provide a secondary side voltage of e.g. 12V AC, which is then used to operate the lamp or lamps. To provide the secondary voltage, electronic type, so-called switching mode power supplies (SMPS) are used. Here, the 50/60 Hz sinusoidal mains voltage is converted to a higher frequency, resulting in smaller transformers to obtain the secondary voltage of e.g. 12V AC.
In the art, various setups of switching mode power supplies are employed to provide the higher frequency. In a so-called self-oscillating power supply, a switching regulator is arranged to alternate the polarity of the rectified mains voltage and thus to provide a high-frequency alternating voltage to a transformer. The secondary side of the transformer is connected to the lamp to provide power to it. The self-oscillating type power supply further comprises a start-up circuit to initiate the switching operation upon connection with power. The oscillation is then self-maintained until the next zero-crossing of the mains voltage, typically using a feedback winding of the transformer to control the switching regulator. A restart is needed at least at each zero-crossing of the mains voltage, which is typically controlled by said start-up circuit of said power supply.
While the above self-oscillating power supply provides a stable operation when used with a common type of lamp, problems arise when using a low-power lamp. Here, the reduced power consumption, although beneficial for aspects of energy efficiency, causes an early disruption of the self-oscillating operation, since the power supply can only maintain the self-oscillation when a defined minimum current is drawn by the lamp. Accordingly, power cannot be provided by the supply to the lamp during substantial periods in each half-cycle of the mains voltage, i.e. between zero-crossings, which can result in an unstable operation, causing e.g. optical flicker.
Corresponding types of self-oscillating power supplies are e.g. disclosed in the publication “Elektronik ecodesign 2007”, WEKA FACHMEDIEN GmbH, Germany, page 30ff.
Accordingly, it is an object of the present invention to provide a circuit arrangement, allowing to efficiently operate a low-power lighting unit with a self-oscillating power supply unit with improved optical and electrical properties.